Electron tube amplifier



Feb. 1@, H942. H. BOUCKE ELECTRON TUBE AMPLIFIER Filed May 28, 1940'lNVl ENTOR w I l-Mllll 1|||| E K a wm BM E w Patented Feb. 10, 1942 I2,272,235 ELECTRON TUBE AMPLIFIER:

Heinz Boucke, Berlin-Charlottenbnrg, Germany,

assignor to Radio Patents Corporation, a corporation of New YorkApplication May 28, 1940, Serial No. 337,606 Germany July 18, 1939 13Claims. .(Cl. 1'l9171) The present invention relates to electron tubeamplifiers, more particularly to amplifiers embodying feedback toimprove the operating characteristics and performance thereof.

It has already become known to reduce or eliminate distortion producedin an electron tube amplifier by the employment of feedback having apredetermined non-linear frequency response.

In order to eliminate or equalize distortion in this a backssubstantially neutralize each other, and

that with increasing signal amplitude th posi-.

tive feedback becomes increasingly predominant over the negativefeedback in such a manner as to eliminate to a more or lesser degreedistortion caused by the upper bend in the characteristic operatingcurve of an amplifying tube and manifesting itself primarily by theformation of a disturbing third harmonic component.

Arrangements of this type have the following advantages among others. Inthe first place, the distortion in a screen grid tube is reducedsomewhat below the corresponding amount of distortion in a single gridtube or ordinary triode with-- out substantially reducing theamplification. Furthermore, the employment of regeneration or positivefeedback does not tend to cause any instability or production ofoscillations, inasmuch as the positive feedback becomes effective onlyrent from an impedance in the plate circuit and deriving negative ordegenerative feedback potential or current from an impedance inserted inthe common cathode vreturn lead of an amplifying tube.

The object of the present invention is to provide an improved feedbackamplifier of the above general type capable of efiicient equalization ofundesirable distortion by the employment of both positive and negativefeedback substantially without sacrifice of amplification.

Other objects and advantages of the invention will become more apparentfrom the following detailed description taken with reference to theaccompanying drawing forming part of this specification and whereinFigure 1 is a circuit diagram of a two-stagetential from at least onescreen grid and means for deriving a negative or degenerative potentialfrom the plate or anode of at least one of the amplifying stages, bothfeedback potentials bein impressed upon the input of the same aredifferent preceding stage. of the amplifier. The design andadjustment'of the circuit is preferably such that for low signalamplitude or small input during excessive control periods or periods ofexcessive signal amplitude and to a comparatively small degree only. Afurther advantage of the inventive method of distortion equalizationwhen used in a push-pull amplifier stage is the fact that in addition tothe third harmonic, also the second harmonic distortion is substantiallycompensated, thus resulting in a practically distortion free amplifier.

Referring to Figure 1, input signal energy-to be. amplified and suppliedby way of input terminals 0-17 is' impressed upon the grid l2 andcathode lliof an electron tube amplifier Ill through a grid couplingcondenser IS in series with an impedance i which may be an ohmicresistance and a grid coupling resistance ll. The tube III which may beof any known type and construction further comprises an anode or plateI3 and is provided with a biasing network I4 constituted by a resistanceand by-pass' condenser inserted in the cathode-to-ground lead to providesuitable grid operating bias in accordance with standard practice.

Amplified signal energy in the plate circuit of tube I0 is impressedupon the-grid 23 and cathode 22 of a subsequent amplifying stageembodying amplifying tube 2| "by way of a resistance cou-. pling networkin the plate circuit comprising a plate coupling resistance l8, gridcoupling condenser l9 and grid leak resistance 20. The tube 2! furthercomprises a screen grid 2| and an anode or plate 25. Amplified signalenergy in the plate circuit of tube 2| may be empressed upon a furthersubsequent stage of amplification potential swing the positive andnegative feed- 65 or may serv to feeds. suitable translating device suchas a loud speaker 34 by way of a coupling transformer 33.

There is further shown in Figure 1 in accordance with the invention anegative feedback circuit comprising a condenser 21 in series with aresistance 28 and connecting the plate 25 of tube 2| to the input sideof grid coupling condenser l9 to provide inverse feedback ordegeneration for distortion elimination and improvement of theoperational stability of the amplifier stage. In addition to the inversefeedback there is further provided positive feedback or regenerationthrough a feedback path comprising a resistance 3|] and a condenser 3|in series and connecting the screen grid 24 of stage 2| to the inputgrid l2 of the preceding stage l0. In order to develop signal potentialvariations for the positive feedback at the screen grid 24, the latteris connected to the source of positive potential indicated by the plussign through a suitable coupling impedance such as a choke coil or anohmic resistance 32 provided in th example illustrated.

In the exemplification according to Figure 1, a two-stage amplifier isrequired to obtain positive feedback from the screen grid of the secondstage to the input grid of the first stage due to the 180 phase reversaleffected in a vacuum tube between the input (grid) and output (plate)potentials. Alternatively, the positive feedback or regeneration may beproduced in the same stage by coupling the screen grid 24 to the inputgrid 23 of the same tube through a properly poled transformer acting asa phase reversing device in a manner well understood.

The coupling resistance 32 in the screen grid circuit or the resistances28 and 30 in the feedback circuits are advantageously made adjustable insuch a manner as to enable a balance of the propagation constants ofboth feedback circuits whereby the positive and negative feedbacksubstantially cancel each other in case of small signal amplitudes orwill result in a slight residual negative feedback for relatively weaksignal energies. As the signal amplitude increases, the positivefeedback from the screen grid will become more and more predominant overthe negative feedback resulting in eflicient compensation orequalization of the distortion caused by the upper bend of thecharacteristic operating curve of the amplifier, that is, primarilyelimination of the third harmonic distortion inherent in vacuum tubeamplifiers.

This phenomenon is due to the fact that, with the proper design andchoice of the circuit constants, the plate current will flatten out athigh amplitude input signal, that is, whenever the input signal is suchas to cause operation on the non-linear portion of the plate operatingcurve a sine wave will be changed into a square wave, while the screencurrent continues to increase substantially linearly even at high inputsignal amplitudes. Accordingly, for low volume of input signal where theplate current operates on the linear portion of the curve and the screencurrent likewise operates on the linear portion of its operating curve,the positive feedback derived from the screen and the negative feedbackderived from the plate will increase and decrease in the same proportionas a function of the input signal amplitude. In accordance with theinvention the circuit constants are chosen in such a manner that thepositive and negative feedbacks are substantially balanced so as tocancel each other whereby distortion-free output without loss in volumeis obtained for relatively low signal amplitudes If, now, the signalamplitude increases beyond the limit where non-linear distortion due tothe curvature or upper bend of the plate current curve occurs, the peaksof the signals in the plate circuit will be flattened. However, thepeaks of the signals developed in the screen grid circuit will not beflattened since the screen current is linear over a wider range than theplate current. Hence, the positive screen feedback output will exceedthe negative plate feedback output, that is, the positive feedback willpredominate over the negative feedback resulting in a correction of thedistortion caused in the plate circuit.

Figure 2 illustrates a single push-pull stage embodying negative andpositive feedback means according to the invention without requiring atransformer or other phase reversing device. There are provided for thispurpose a pair of screen grid vacuum tubes 40, 40 having cathodes 4|,4|, input grids 42, 42, screen grids 43, 43' and anodes or plates 44,44', respectively. Signal energy to be amplified delivered by way ofinput terminals 11-12 is impressed upon the primary 45 of an inputtransformer having a pair of secondaries 46, 46', the former beingcoupled to the grid 42 and cathode 4! of tube 40 in series with voltagedivider resistances 4'! and 48 and the latter being coupled to the grid42' and cathode 4l' of tube 40 in series with voltage dividerresistances 4'! and 48', respectively. Items 50 and 50' are networksinserted in the cathode-to ground leads of the tubes to provide suitablegrid operating bias in accordance with standard practice. The plates 44and 44 of the tubes are connected to the opposite ends of the outputtransformer primary 5| having a secondary 52 feeding in the exampleshown a loud speaker 54. The center point of the primary 5| is connectedin a known manner to the positive pole of a. suitable source of spacecurrent 53 having its negative pole connected to ground, or any otherzero potential point of the system.

In order to avoid the use of transformers or other phase reversingdevices, the positive feedback derived from the screen grid of one tubeis impressed upon the input grid of the other tube this being enabled inview of the operation of the push-pull circuit in which the potentialsat the respective points of the two circuits are in phase opposition toeach other. The screen grid potentials are produced by suitable couplingimpedances such as resistances 56 and 56 connecting the screen gridswith the positive pole of the source 53. Positive feedback potential forthe tube 40 is derived from the screen grid 43 of the tube 40 through afeedback circuit including a condenser 60' and resistance GI andconnecting the screen grid 43 with the cathode side of the secondarywinding 46 of the input transformer 45. Similarly, positive feedbackpotential for the tube 40 is derived from the screen grid 43' of tube40' through a feedback path comprising condenser 60 and resistance BIand connecting the screen grid 43 with the cathode side of the secondarywinding-46 of the input transformer 45.

The negative feedback is produced in the manner similar to thataccording to Figure 1 by the provision of feedback paths includingcondenserresistance series network 5859 and condenserresistance seriesnetwork 58'--59 and connecting the plates 44 and 44"to the junctionpoints between the voltage divider resistances I1, 48 and 41', 4.8,respectively. Like in'Figure 1, it is advantageous in a system of thistype to provide oneor more adjustable impedances such as the screen gridresistances B-and 56' or the feedback resistances 59,59 and 6|, 6|, oralternatively, the feedback condensers 58, 58' or 60,

60' for properly balancing the positive and negative feedback in themanner described. The circuits, both according to Figure 1 and Figure 2,

are preferably designed in such a manner that of the first stage,further means for feeding back degenerative I signal potential from theplate to the grid of the second stage, and means whereby theregenerative and degenerative feedbacks cancel each other for apredetermined signal amplitude and the regenerative feedback exceeds thedegenerative feedback in proportion to. an increase insignal amplitudebeyond said predetermined amplitude.

4. A resistance coupled amplifier comprising an input and an output andat least two amplifying the screen grid resistors 56 and 56' have assmall a range as will'be compatible with the function and eflicientoperation of the system.

If a reduction of the amplification is not objectionable the negativefeedback may be ad- Justed to substantially exceed the positive feedbackin which case both the second and third harmonic distortion will besubstantially eliminated resulting in asubstantially distortionfreeamplifier.

It will be furthermore evident that the coupling circuits may bedesigned to have a desired stages in cascade, a coupling impedance inthe screen grid circuit of the second stage, a-first frequency responseand/or may be of any suitable type known in order to ensure a constantphase especially with regard to the negative feedback potential forsubstantially all the frequency components of the wave energy beingtranslated or amplified. l

From the foregoing it will be evident that the invention is not limitedto th specific details and design of circuits shown (1 disclosed hereinfor illustration but that the underlying idea regenerative feedback pathconnecting the screen grid of the second stage to the input of the firststage, a second degenerative feedback circuit connecting the plate andgrid of the second stage,

and means whereby said regenerative and degenerative feedbacks canceleach other for a predetermined signal amplitude and, the regenerativefeedback exceeds the degenerative feedback in proportion to an increasein signal amplitude beyond said predetermined amplitude.

5. A resistance coupled amplifier comprising an input and an output andat least two stages in cascade the second of which embodies a screengrid amplifying tube, coupling impedance means in the screen gridcircuit for, the second stage,

. a first regenerative feedback circuit including a condenser andresistance in series connecting the screen grid of the second stage tothe input of the first stage, a second degenerative feedback circuitincluding a condenser and resistance in and principle of the inventionwill be susceptible of numerous yariations and modifications comingwithin its broader scope'and spirit as defined in the appended claims.The specification and drawing are accordingly to be regarded in anillustrative rather than a limiting sense.

I claim: 1 An electron tube amplifier comprising an input and an outputand at least one screen grid amplifying tube, means for regenerativel'yfeeding back screen grid signal potential from a higher to a lowerenergy level of said amplifier, further means for .degenerativelyfeeding back plate 'signal potential from a higher to a lower energylevel of said amplifier, and means whereb the regenerative anddegenerative feedbacks cancel each other for a predetermined signalamplitude and the regenerative feedback exceeds the degenerativefeedback in proportion to an increase in signal amplitude beyond saidpredetermined amplitude.

2. A resistance coupled electron tube'amplifier comprising an input andan output and at least two stages in cascade the second of whichembodies a screen grid amplifying-tube, means for feeding backregenerative potential from the.

screen grid of the second stage to the input of the first stage, furthermeans for feeding back degenerative potentialfrom the output to theinput of one of said stages, and means whereby the regenerative anddegenerative feedb'acks cancel each other for a predetermined signalamplitude and the regenerative feedback exceeds the degenerativefeedback in proportion to an increase of signal amplitude-beyond saidpredetermined amplitude.

3. Aresistance coupled electron tube amplifier comprising an input andan output and at least two stages in cascade the second of whichembodies a screen grid amplifying tube, means. for feeding backregenerative signal potential from the screen grid of the second stageto the input series connecting the plate andgrid of at least one of saidstages, and means whereby the regenerative and degenerative feedbackscancel each other for a predetermined signal amplitude and theregenerative feedback exceeds the degenerative feedback in proportion toan increase in signal amplitude beyond said predetermined amplitude.

6. The combination with a push-pull amplifier comprising a pair ofelectron tubes each having a cathode, an input grid, a scr'een grid anda plate atively connected therewith, of load impedancemeans in thescreen grid circuits of said tubes, regenerative feedbackcircuitscoupling the screen grid of each tube with the input of the other tube,and degenerative feedback circuits coupling the plate of each tube withthe input of the same tube.- I

8. The combination with a push-pull amplifier comprising'a pair ofelectron tubes each having a cathode, an input grid; a screen grid and aplate and input and output circuits operatively connected therewith, ofload impedance means in the screen grid circuits of each tube, apotential divider in the input circuit of each tube,

a pair of regenerative feedback circuits connecting the screen grids ofeach tube to a point on the potential divider in the input circuit ofthe other tube, and a pair of degenerative feedback-p circuitsconnecting the plates of each tube to.

another point on the potential divider in the input circuit of the sametube.

9. The combination with a push-pull amplifier comprising a pair ofelectron tubes each having a cathode, an input grid, a screen grid and aplate and input and output circuits operatively connected therewith, ofa potential divider in the input circuit of each tube, load impedancemeans in the screen grid circuit of each tube, a pair of regenerativefeedback circuits comprising a condenser and resistance in series andconnecting the screen grid of each tube to a point on the potentialdivider in the input circuit of the other tube, and a pair ofdegenerative feedback circuits comprising a resistance and a condenserin series and connecting the plate of each tube to another point on thepotential divider in the input circuit of the same tube.

10. The combination with an alternating current electron tube amplifier,of a positive feedback circuit connected between points of higher andlower energy level of said amplifier, a negative feedback circuitconnected between points of higher and lower energy level of. saidamplifier, the positive feedback balancing the negative feedback below apredetermined amplitude limit of input energy applied to said amplifierabove which non-linear distortion occurs due to saturation of said tube,and means whereby the positive feedback increases with respect to thenegative feedback as the amplitude of the input energy exceeds saidpredetermined amplitude to compensate for said non-linear distortion.

11. The combination with an alternating current electron tube amplifierincluding at least one screen grid stage, of a positive feedback circuitconnected between the screen of said stage and a point of lower energylevel of the amplifier, a

further negative feedback circuit connected between the plate of a stageand a point of lower energy level of the amplifier, the operatingconstants of said tube and said feedback circuits being s0 adjusted thatthe positive feedback balances the negative feedback up to apredetermined amplitude of input energy applied to said amplifier beyondwhich nonlinear distortion occurs due to saturation of said tube, andthat the positive feedback increases with respect to the negativefeedback as the amplitude of the input energy exceeds said predeterminedamplitude to compensate for said non-linear distortion.

12. The combination with an alternating current electron tube amplifier,of a positive feedback circuit connected between points of higher andlower energy level of said amplifier, and means to normally render saidfeedback ineffective for input signal amplitudes below a predeterminedlimit beyond which non-linear distor tion of the amplifier occurs due tosaturation of said tube and to increase said feedback as the signalamplitude exceeds said predetermined amplitude to compensate for saidncn-linear distortion.

13. An electron tube amplifier comprising an input and an output and atleast one screen grid amplifying stage, means for regeneratively feedingback screen grid signal potential to a point of lower energy level'ofthe amplifier and further means for degeneratively feeding back platesignal potential from a pointof higher to a point of lower energy levelof said amplifier, the regenerative and degenerative feedbacks beingbalanced for input signal amplitudes below a predetermined limitamplitude.

. HEINZ BOUCKE.

